[PATCH] D36586: [clang-tidy] hicpp bitwise operations on signed integers

[Jonas Toth via Phabricator via cfe-commits]

JonasToth added inline comments.


================
Comment at: clang-tidy/hicpp/SignedBitwiseCheck.cpp:23
+  const auto SignedIntegerOperand =
+      expr(ignoringImpCasts(hasType(isSignedInteger()))).bind("signed_operand");
+
----------------
aaron.ballman wrote:
> JonasToth wrote:
> > aaron.ballman wrote:
> > > JonasToth wrote:
> > > > aaron.ballman wrote:
> > > > > Is ignoring implicit casts the correct behavior here as far as the coding standard is concerned? Consider:
> > > > > ```
> > > > > unsigned char c = 128;
> > > > > f(~c); // c promotes to int before ~ is applied
> > > > > ```
> > > > > Looking at the coding standard, I get the impression this is not allowed, but I'm not *super* familiar with HIC++.
> > > > I first implemented it without ignoring the implicit integer casts, the result was, that most cases (in test cases) where not found. therefore i implemented it that way. I add an testcase for this and see how i need to adjust the matcher.
> > > > 
> > > > Could you help me there with the semantic, since i am not so fluent in C/C++ standardese, but i think the findings are reasonable.
> > > It kind of boils down to the intention from the HIC++. Consider a test case like:
> > > ```
> > > void f(int i);
> > > 
> > > void g() {
> > >   unsigned char c = 127;
> > >   f(~c);
> > > }
> > > 
> > > ```
> > > Does `f()` expect to receive `-128` or `128`? I think this code will pass your check (ignoring the promotion means the type is `unsigned char`), but the actual bitwise operation is on a signed integer value because there is an integer promotion. So 127 is promoted to int, then ~ is applied, resulting in the value `-128` being passed to the function.
> > Yeah i see, i have such cases added in the tests.
> > TBH. i don't know if the standard wants this covered, but the demonstrated case is definitly bad.
> > 
> > Would it be a good idea, to warn on assigning/initializing `signed` integers with `unsigned` integers?
> > 
> > The CppCoreGuidelines have some sections on that as well: [[ https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#arithmetic | Section ES.100-103 ]]
> > 
> > Not sure if this check should care. On the other hand, would it  be nice to have a check that covers all "integer problems".
> > Yeah i see, i have such cases added in the tests.
> > TBH. i don't know if the standard wants this covered, but the demonstrated case is definitly bad.
> 
> I think you should ask the HIC++ people what they think; the rule text does not make it clear what the behavior should be here.
> 
> > Would it be a good idea, to warn on assigning/initializing signed integers with unsigned integers?
> 
> We already have such a warning in clang (-Wsign-conversion).
> 
> >The CppCoreGuidelines have some sections on that as well: Section ES.100-103
> >
> >Not sure if this check should care. On the other hand, would it be nice to have a check that covers all "integer problems".
> 
> Any such check will require so many options that it is likely to be almost unusable. However, I would not be opposed to seeing a clang-tidy module that has a bunch of checks in it that relate to integer problems.
i think, those could land in `bugprone-`, and be aliases to hicpp and the cppcoreguidelines if appropriate.

depending on my time (i should have a lot of free time for 2 months) 
i will try to implement some.
is there a resource with all common problems found with integers?


https://reviews.llvm.org/D36586